1. Field of the Invention
This invention relates to a medical device, and particularly to a medical device capable of withstanding sterilization by radiation. More particularly, this invention relates to a medical device such as the tube for an administration set, which is made of vinyl chloride resin and, therefore, is capable of withstanding sterilization by radiation.
2. Description of Prior Arts
Heretofore, it has been customary for such medical devices as blood bag, solution administration set and catheter to be first sterilized as by the wet sterilization or the action of a sterilizing agent such as ethylene oxide gas and then stowed in respective packages similarly sterilized in advance or for them to be first contained in respective packages and thereafter subjected in situ to sterilization. In the case of the thermal sterilization, however, there has been entailed the problem that the synthetic resin used in such medical devices is required to offer heat resistance to the heat of sterilization and yield sparingly to thermal deformation. In the case of the sterilization with ethylene oxide gas, there has been involved the problem that, after the sterilization, a good deal of time is wasted before the sterilized medical devices will be free from ethylene oxide gas and then ready for use.
As a method for effecting required sterilization without requiring such use of heat or sterilizing agent, the method of sterilization by radiation has been recently proposed. Since this method is carried out at low temperatures, it no longer entails the requirement that the materials of medical devices should be capable of withstanding heat and shunning thermal deformation or the problem regarding the manner of handling such medical devices. The use of radiation nevertheless causes irradiated medical devices to suffer from adverse effects such as deterioration and discoloration. Particularly, medical devices made of or using flexible vinyl chloride resin, when sterilized by radiation entail the problem that they fail in the safety test which all medical devices are required to pass. For example, the vinyl chloride resin composition which has found popular acceptance for use in medical devices comprises 100 parts by weight of polyvinyl chloride, 30 to 80 parts by weight of di-2-ethylhexyl phthalate, about 0.01 to 5 parts by weight of a calcium-zinc type metallic soap as a stabilizer, about 0 to 10 parts by weight of epoxidized soybean oil as an assistant stabilizer, and optionally about 0 to 5 parts by weight of slidant. When a molded article of such flexible vinyl chloride resin was sterilized by exposure to radiation and thereafter tested by the "Standard for Blood Bag Set made of Vinyl Chloride Resin" (Ministry of Health and Welfare Standard), Notification No. SHOWA 40(1965)-448 of Ministry of Health and Welfare or the "Method for Testing Plastic Containers for Transfusion (Pharmacopieial Standard)" of the Tenth Amended General Testing Methods, No. 42, of the Japaneses Parmacopoeia to determine whether or not the molded article would discharge any extract and whether the extract, if any, would cause hemolysis, the results obtained by the irradiation at a dosage of 3 Mrads were found to exceed the tolerable limits. Thus, the resin proved to be rejectable as a material for medical devices destined to be sterilized by radiation. When the extract from the molded article was subjected to the cell toxicity test designed to ensure more reliable safety of containers, it was found to possess discernible toxicity.
A stabilizer such as zinc stearate, calcium stearate or magnesium stearate has hitherto been incorporated with the vinyl chloride resin. However, the amount of the stabilizer is larger. Such stabilizer also acts as a lubricant, so it bleeds out on the surface during the molding.
As the resin composition for medical use, a composition comprising 100 parts by weight of a vinyl chloride resin, 5 to 200 parts by weight of an ethylene-carbon monoxide-vinyl acetate copolymer and 0.001 to 10 parts by weight of powdered magnesium oxide or calcium oxide is known (Japanese Patent Open No. Sho 56(1981)-41240). The ethylene-carbon monoxide-vinyl acetate copolymer as a softening agent is apt to yield acetic acid during heat molding and the acetic acid is neutralized by megnesium oxide or calcium oxide. However, the above composition can be molded only below 120.degree. C. If the composition is heated above 120.degree. C., diacetic acid is enhanced. Further, if the medical device made of the composition is sterilized by radiation, acetic acid is apt to be produced. Thus if acetic acid is liberated, it will be extracted into medical solution or blood.